This invention relates to a process for the stereo-selective preparation of 8.alpha.,12-oxido-13,14,15,16-tetranorlabdane and to the use of aluminium oxide having certain properties for the stereoselective preparation of 8.alpha.,12-oxido-13,14,15,16-tetranorlabdane.
1. Field of the Invention
8.alpha.,12-Oxido-13,14,15,16-tetranorlabdane 2 (Ambroxan.RTM.) is an extremely valuable ambergris fragrance which is found in ambergris, a metabolic secretion of the sperm of whale (Ullmann's Encyklopddie der technischen Chemie, Vol. 20, page 283, Verlag Chemie Weinheim 1981). Ambroxan.RTM. may be synthesized from sclareol by oxidative side chain degradation and subsequent reduction of the lactone formed (sclareolide) in accordance with U.S. Pat. No. 3,050,532. The sclareolide is converted into the odorless diol, 8.alpha.,12-dihydroxy-13,14,15,16-tetranorlabdane 1, in known manner, for example by reduction with lithium aluminium hydride (Helv. Chim. Acta 33, 1310 (1950)), with sodium borohydride (Chem. Abstr. 57, 7316a) or with potassium borohydride/lithium chloride mixtures (Chem. Abstr. 94, 15913q). ##STR1## The dehydration of diol 1 with ring closure to Ambroxan.RTM. may be carried out with acidic catalysts, for example p-toluene sulfonic acid, p-sulfonic acid chloride, catalytic quantities of sulfuric acid and also acidic ion exchangers in various solvents, for example toluene, hexane, pyridine, tetrahydrofuran or methanol, preferably at boiling temperature. These processes frequently give mixtures which, in addition to unsaturated alcohols, olefins and the substantially odorless 8-epi-oxide 2a (Helv. Chim. Acta. 68, 2022 (1985)), contain the odor-active, equatorially 8.alpha.,12-oxido-linked furan 2, in quantities of only 20 to 50% by weight.
U.S. Pat. No. 3,029,255 describes the use of .beta.-naphthalene sulfonic acid or alumina as dehydration catalysts in the production of Ambroxan.RTM.. In addition to resinification products and olefins, other secondary products are formed in this process, so that the yields of Ambroxan.RTM. are less than 77%.
It is known from JP 61/33184, reported in Chem. Abstr. 105, 134193k (1986), that the theoretical yields of Ambroxan.RTM. can be increased to 85-90.5% by using active white earth, alumina or silica charged with 1 to 20% by weight sulfuric acid, phosphoric acid or polyphosphoric acid. In two Examples, the purity, i.e. the content of odor-active furan 2, is put at 97% and 98%, respectively.
The problem addressed by the present invention was to develop a process for the stereoselective preparation of 8.alpha.,12-oxido-13,14,15,16-tetranorlabdane which would enable the yield of Ambroxan.RTM. with a purity of &gt;95% to be increased to considerably more than 90%.